A femtosecond laser light source generates an ultrashort pulse having a high peak power and has a high average output of generated pulses. Examples of such a femtosecond laser light source include a femtosecond (fs) laser pulse. The femtosecond laser light sources have been broadly applied not only to basic science fields including ultrahigh speed aspectrochemistry, high energy physics, XUV-wave generation and the like but also to a variety of fields including microprecision laser processing, micro-surgery.
Generally, the femtosecond laser pulse has good properties. Examples of the good properties include a short pulse time width, a high peaking power and a broad spectrum bandwidth.
Such the femtosecond laser pulse may be applied to micro or nano processing of electronic components and optical components requiring ultraprecision. Examples of such electronic and optical components include a solar cell, an optical memory, a semiconductor and a flat panel display. Accordingly, demands on femtosecond pulse laser system for industry have been increasing.
To meet such demands on the femtosecond pulse laser system, conditions for applying the femtosecond laser pulse to the ultra precision laser processing will be described.
First of all, a laser pulse time width is much shorter than an electronphonon relaxation time of an object not to transmit a thermal energy generated in processing to a portion near a portion that will be processed (non-thermal processing).
That is called as cold ablation.
For example, the electronphonon relaxation time of aluminum is 4.27 picosecond (hereinafter, ps) and that of iron is 3.5 ps and that of copper is 57.5 ps.
Specifically, when femtosecond laser processing aluminum, laser pulse are applied to aluminum in a pulse time width that is a picoseconds or less for cold ablation.
Accordingly, the femtosecond laser is proper to the ultraprecision laser processing of cold ablation.
A femtosecond laser pulse in a femtosecond range can minimize thermal diffusion in a processing region and not cause the damage generated by latent heat. Accordingly, the femtosecond laser can process quite a hard material and have a short pulse time width as well as high pulse energy, with a high peak power, only to be advantageous to a nonlinear optical effect, namely, multi-photon absorption. Accordingly, the femtosecond laser can perform various nanometer scaled ultra precision processing of various materials. Examples of such various materials include glass, polymer and even transparent materials.
Second, processing object materials have an ablation threshold values that are approximately several J/cm2 or more. Considering the size of the laser beam concentrated on a processing area for ablation processing, a pulse energy of approximately 10 μJ is required.
In some cases of material processing application can require hundreds of μJ pulse energy.
An exemplary one of lasers having those good properties may be titanium sapphire laser (Ti: sapphire laser).
Until now, a commercially useful Ti sapphire laser may provide approximately several to hundreds of femtosecond pulse time width, several mJ or several J pulse energy.
However, in a conventional femtosecond laser apparatus, a high-priced high output pulse green laser such as Nd:TVO4 laser has to be used as a pumping laser source and it is difficult to gain dozens of kHz or more pulse repetition rate.
Also, the Ti sapphire laser has a large scale system and a high price and also it is difficult to maintain a pulse power stably and it is not easy to use the Ti sapphire laser in a production worksite.
Meanwhile, a diode-pumped solid-state (DPSS) laser uses a micro-sized light source as a pump beam source. Examples of such a micro-sized light source include a laser diode. As a femtosecond laser is configurated by using a solid laser material, an optical pumping structure can be simple and the size of a laser head can be small. Also, a laser diode at a wavelength commercially used in various fields has a relatively low price in comparison to the power thereof and the price of the femtosecond laser can be reduced, such that an effect of cost reduction can be gained.
In addition, the solid laser has a short optical pumping distance and it can perform stable laser operation, such that it can be advantageously applied to a laser for industrial usage.
Recently, with development of semiconductor and electronic technologies, a laser diode array, a laser diode bar and the like have been developed that are able to perform high power, with small sizes and high efficiency, and development of solid laser systems using diode pumping have been growing rapidly.
To realize the femtosecond laser system that is able to optically pump, using a laser diode, it is necessary to select a laser material meeting predetermined conditions and to design and fabricate an optical pumping module for performing the optical pumping.
Crystals doped with rare-earth ions that can perform diode pumping at 808 nm and 980 nm, respectively, may be usually used as the laser material for diode pumping. Examples of such crystals doped with rare-earth ion may include Neodymium (Nd) and Ytterbium (Yb).
In an early stage of a high power laser developing step, a laser crystal doped with Neodymium is preferred because of a 4-level structure and various absorption lines. In recent, a laser crystal doped with Ytterbium has been used a lot because of excellent thermal and optical properties.
There are additional necessary conditions required to apply the femtosecond laser light source to industrial settings including the ultraprecision laser processing.
For example, if a pulse repetition rate of a laser is low, it takes much time to perform laser processing and productivity of the industrial settings might deteriorate.
However, it is preferred to heighten the pulse repetition rate of the laser but it is restricted to height the pulse repetition rate.
If the pulse repetition rate is too high to receive the next laser pulse before plasma generated by the femtosecond laser pulse disappears, the next laser pulse can be ill-affected by the plasma remaining near a target. For example, a beam traveling direction of the next laser pulse might be changed or the pulse time width thereof might be changed.
That phenomenon is called as ‘plasma shielding’.
To restrain the plasma shielding, the next laser pulse has to be applied after a relaxation time of the former plasma.
In other words, a time interval between one laser pulse and the next laser pulse has to be longer than the plasma relaxation time. The plasma relaxation time may be different for each of processing object materials. However, the repetition rate of the plasma relaxation time is approximately 1 MHz based on the laser pulse repetition rate.
Accordingly, to maintain the high productivity in industrial settings, a femtosecond laser having a pulse repetition rate at hundreds of kHz is required.
In addition, to mount a laser light source to a laser processing system and to operate the laser processing system, the femtosecond laser is required to have a compact size and a low price and a high operation stability that makes a laser operational state not changed for a substantially long time.
When a femtosecond pulse is generated in mode locking in a femtosecond oscillator initially, a pulse energy of the femtosecond pulse is lowered by a nanojoule (nJ) and it is not appropriate to apply such the femtosecond pulse to a laser processing.
To enhance the femtosecond pulse energy, Chirped Pulse Amplification (CPA) is used.
For example, a pulse stretcher is used in stretching a pulse generated from a femtosecond oscillator longitudinally and timely and the pulse is applied to an amplifier to amplify the pulse energy.
Hence, the amplified pulse passes a pulse compressor to restitute a time width of the pulse to an initial femtosecond range.
At this time, the pulses generated from the oscillator are employed as seeding pulses applied to the amplifier.
Pulses are timely and longitudinally stretched by a difference of passages according to wavelengths generated in the pulse stretcher, which can be called as ‘chirping’ and technology for amplifying the pulse energy is called as ‘chirp pulse amplification technology’.
When using such chirp pulse amplification technology, a peaking power of the pulse amplified in a resonance cavity of the pulse amplifier kept low and non-linear distortion generated in spatial or temporal distribution of the laser pulses can be retrained. Moreover, a physical damage applied to optical components composing the system can be prevented.
Specifically, the pulse amplifier can be operated to prevent the damage to the system generated by the high energy laser pulse and to enhance the pulse energy.
Recently, the high pulse energy has been gained from MOPA system combined with Maser Oscillator (MO) and Power Amplifier (PA) and a huge step has made in development of femtosecond laser systems having a high peaking power and a high average power accordingly. The master oscillator directly pumps a diode light source based on the chirp pulse amplification technology and the power amplifier directly pumps the diode light source.
However, the laser material doped with Ytterbium has a 2-level energy structure or a 3-level energy structure and it has a disadvantage that lights emitted at an optical pumping wavelength of 981 nm is absorbed by the laser material again.
To solve the disadvantage, the light generated from a high brightness laser diode having a high power is focused on the laser crystal with a micro-spot size.
In this process, pump beams failed to be converted into laser beams are transmitted adjacent to the spot of the laser crystal in a thermal energy type and to a mount where the laser crystal is mounted.
When the thermal energy is collected largely, amplified laser beams are crushed and the quality of beams might deteriorate. Also, an average power of lasers and pulse energy might be restricted.
After that, the thermal energy focused on the laser crystal is higher than a damage threshold, the laser crystal might have physical damage, for example, crack or break only to stop laser oscillation problematically.
Meanwhile, conventional studies on generation or amplification of femtosecond pulses by using a plurality of Yb:KYW or Yb:KGW laser crystals will be described as follows.
For example, U.S. Pat. No. 7,508,847 B2 discloses a new concept of increasing the frequency of passages of laser beams via gain material in a resonance cavity by pumping two anisotropic materials including Yb:KGW.
However, in the application, the length of the resonance cavity is increased to reduce an instable phenomenon of a pulse power called as “Triggered mode”. It is proposed that a pumping structure of two gain materials using one of various types of long resonance cavities. However, that application fails to experimental embodiments and results.
In addition, U.S. Pat. No. 6,760,356 B2 discloses a method of amplifying a femtosecond pulse by using two anisotropic laser crystals of Yb:YAG.
However, those two prior patent applications disclose only the power amplification by using only two laser materials and fail to disclose thermal, optical and other various characteristics that have to be put into consideration of an optical axis of a laser material in the other femtosecond laser systems. In other words, the two patent applications disclose only an effect of the output power enhanced by increasing the numbers of the laser materials, with no comments on the axis of the laser material.
Meanwhile, U.S. Pat. No. 6,891,876 B2 discloses a study on absorbing depolarized pump beams by selecting an axis of an Yb:KYW laser material and controlling a polarization rate of lights pumping a laser material.
The application focuses on light pumping of an anisotropic material. Even in case of using a first depolarized pump beam, an axis of an anisotropic gain material and a wavelength of a pumping laser are selected to optically pump the laser material efficiently.
The application discloses that even in case the wavelength of a second pump beam is instable, an axis of the laser material is selected to optically pump the laser material efficiently.
The proposed method uses an absorption spectrum that is differentiated according to an axis direction of the laser material and properly determines a direction of the laser material based on the different absorption spectrums to properly mix two axes of one laser material. When the intensity of the pump beam is controlled according to the polarization direction, similar absorption cross-sections are shown in broad wavelength region.
However, in this instance, there is an advantage of providing a light pumping unit less sensitive to instability of polarization or wavelength of the pump beam. However, to have similar absorption cross-sections in broad wavelength regions, an absorption cross-section of the laser material has to perform light pumping in a wavelength region having a small absorption cross-section of the laser material.
Moreover, the output has to be dispersed to two polarized lights from one laser light source. Because of that, pumping efficiency could deteriorate badly and the pump beams absorbed after failed to be converted into laser wavelengths are focused on the laser material by the terminal energy, such that the quality of the laser beam may deteriorate and the power of the laser beam may be restricted disadvantageously.
The Yb:KYW or Yb:KGW laser crystal has a good heat conduction quality and it has an advantage of generating a high average power femtosecond laser. However, the conductivity of the anisotropy laser material such as Yb:KYW or Yb:KGW is differentiated according to the axis direction. If the average power of the laser is high, astigmatism of a thermal lens might be generated by a thermal effect and the shape of the laser beam is distorted to deteriorate the quality of the laser beam.
In addition, while generating or amplifying the femtosecond pulse by using the plurality of the laser crystals, the pump beam generated from the laser diode is incident on and absorbed to one of the laser crystals and the other pump beams not absorbed to the laser crystal are irradiated to optical components or optical mounts. Accordingly, the alignment of the pump beams and laser beams might deteriorate.